U.S. Pat. No. 3,269,466 discloses a hammer drill having a single shaft intermediate the output shaft of the motor and the hollow spindle, carrying both a bevel gear to rotate the spindle and a crank member to drive the piston of the hammer mechanism, the intermediate shaft being perpendicular to the axis of the spindle due to the use of a crank member. Such a single intermediate shaft provides the dual advantages of compactness and reduced weight, compared to a conventional hammer drill having two, intermediate shafts (one shaft to rotate the spindle and the other shaft to rotate the crank member). In the hammer drill of U.S. Pat. No. 3,269,466, because the crank member drives the piston inside the hollow spindle, and the bevel gear on the intermediate shaft engages the bevel gear on the hollow spindle, the distance between the crank member and the bevel gear on the intermediate shaft is tightly constrained, with the bevel gear located between the crank member and the engagement with the output shaft of the motor. Because the intermediate shaft causes both the hollow spindle and the crank member to rotate, the drill inevitably always performs both drilling (rotational) and hammering (longitudinal) actions in use. In order to choose whether an attached tool bit is only rotated, only hammered, or both rotated and hammered, the user must choose a tool bit having an appropriate design of shank. However, regardless of the design of tool bit shank, the internal mechanism of the hammer drill always performs both rotational and hammering actions when when the motor is operated. Consequently, when only one, of the actions is required, the energy expended on the other action is wasted, and unnecessary vibrations are produced.
U.S. Pat. No. 4,895,212 discloses a hammer drill having a single shaft intermediate the output shaft of the motor and the hollow spindle, the intermediate shaft carrying both a gear to rotate the spindle and a wobble drive to cause the piston of the hammer mechanism to reciprocate, and the intermediate shaft being parallel to the axis of the spindle due to the use of a wobble drive. As is conventional, the wobble drive (comprising a rotational hub carrying an obliquely arranged arm on a bearing arrangement) is located adjacent to the motor output spindle so as to be behind the hollow spindle, and thus the gear on the intermediate shaft which rotates the hollow spindle is located on the opposite side of the wobble drive to the engagement with the output shaft of the motor. Consequently, in the hammer drill of U.S. Pat. No. 4,895,212 the intermediate shaft can be sufficiently long, between the wobble drive and the gear which rotates the spindle, for a mode change mechanism to be provided, to enable the wobble drive to be turned on or off.
European Patent Application EP 1857228 A1 discloses a mode change mechanism for a hammer or percussion drill driver of the type in which the hammer or percussion action is provided by a ratchet mechanism rather than by a piston driven by a crank member or wobble drive. The mode change mechanism includes a rotatable collar having a cam surface which moves an elongate member when the collar is rotated, to change the operating mode of the drill driver.
U.S. Pat. No. 5,435,397 discloses a hammer drill having a hollow spindle and a hammer mechanism including a piston driven by a wobble drive. The hammer drill includes a rotatable collar which opens and closes vent openings in the hollow spindle, to vary the impact energy provided by the pneumatic hammer mechanism.